In a coder/decoder (codec) there are typically two channels, a digital to analog transmit (TX) channel and an analog to digital receive (RX) channel. The TX channel contains a digital to analog converter (DAC) along with analog and digital signal conditioning circuits. The RX channel includes an analog to digital converter (ADC). Both of the DAC and ADC circuit blocks require a fundamental reference input, which may be a voltage, a current, or other input.
When looking into these blocks from a very high level perspective, the DAC output is the digital code multiplied by the reference and the ADC output is the input signal divided by the reference. This means that the DAC and ADC can only be as high performance as its reference. As a consequence of this fact, any non-linearity or inaccuracy in the reference will be propagated to the output of the DAC or ADC block, which makes the reference block a fundamental item in the codec.
The standard approach in full-duplex codecs is to use two reference buffers to isolate the reference of the DAC and ADC. This is a costly strategy because, in the case of a voltage reference, the two voltage buffers alone can represent more then 20% of the analog power consumption of the codec and up to 11% of the analog core area.